Rasul Golam, Prakash G K Surya, Olah George A
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, University Park, Los Angeles, California 90089-1661, USA.
J Phys Chem A. 2006 Oct 5;110(39):11320-3. doi: 10.1021/jp060800h.
A comparative study of the hypercoordinate square-pyramidal carbocations C7H9+ and C8H9+ was performed by the ab initio/GIAO-CCSD(T) method. The structures and 13C NMR chemical shifts of the cations were calculated at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. The bishomo square pyramidal structure 1 was calculated for C7H9+ at the MP2/cc-pVTZ level. The calculated 13C NMR chemical shifts of structure 1 agree extremely well with the experimental values. However, unlike for C7H9+ both the bishomo square pyramidal structure 3 and the trishomocyclopropenium type structure 4 were found to be minima on the potential energy surface of C8H9+. They are very close energetically with cation 3, only 0.7 kcal/mol less stable than cation 4 at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level. Neither structure 3 nor 4 yields NMR spectra that agree with experiment. However, a weighted average of the two reproduces the observed NMR spectrum of C8H9+ (at -80 degrees C) quite well.
采用从头算/GIAO-CCSD(T)方法对超配位四方锥碳正离子C7H9+和C8H9+进行了比较研究。在GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ水平上计算了阳离子的结构和13C NMR化学位移。在MP2/cc-pVTZ水平上计算得到C7H9+的双同型四方锥结构1。结构1计算得到的13C NMR化学位移与实验值极为吻合。然而，与C7H9+不同的是，在C8H9+的势能面上，双同型四方锥结构3和三同型环丙烯型结构4均为极小值。它们在能量上非常接近，在MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE水平下，阳离子3仅比阳离子4稳定0.7 kcal/mol。结构3和4都不能给出与实验相符的NMR谱。然而，两者的加权平均值能很好地重现C8H9+（在-80℃下）的实测NMR谱。
Zotero 插件